Orientation in mono substituted ring systems

Aromatic electrophilic substitution
presented By Dalpat Singh

1. Orientation in Mono substituted ring systems Aromatic Electrophilic Substitution Presented By :- Dalpat singh MSc 2 nd Sem . Submitted to :- Dr. M.R.K. Sherwani

2. Introduction :- When a monosubstituted benzene undergoes an electrophilic substitution , the position taken up by the incoming group (The Orientation) and the rate of reaction ( the reactivity ) are determined by the substituent already present on the benzene ring. On the basis of various substituents can be divided into three categories :- Ortho , para-directing and activating groups : if an electron donating group is already present on the benzene ring then the incoming group is attached to the ortho and para positions. these type of substituted benzene rings are more reactive then the benzene itself. This is called activation of benzene ring. Ex :- NR 2 , NHR , NH 2 ,OH ,OR ,NHCOOR etc. Meta-directing and deactivating Groups :- if an electron withdrawing group is already present on the benzene ring then the incoming group is attached to the meta positions. These type of substituted benzene rings are less reactive towards the electrophilic substitution than benzene itself. This is called deactivation of benzene ring systems. Ex :- NO 2 , NR 3 , CF 3 , CN , CHO ,COR , etc . Ortho, para-directing and deactivating groups :- halogens are Ortho and para directing but they deactivate the benzene ring for further electrophilic substitution.

3. Explanation based on charge distribution : Ortho, Para-directing and activating Groups : (a) when an atom having a lone pair of electron directly attached to the ring then the electron density on ortho and para position increased through conjugation. (b) Aryl groups are also ortho and para directing and activating groups because they increase the electron density on o- and p- positions as follows :-

4. (c) in case of alkyl groups the electron density on ortho and para positions is increased by hyperconjugative effect. Thus, they are ortho and para directing and activating groups. 2. Meta-directing and deactivating groups : (a) when an electron withdrawing group directly attached to the ring then the electron density on ortho and para position decreased through conjugation, thus, m-positions have a relatively high electron density. Therefore, further electrophilic substitution is made more difficult by the presence of a m-directing group.

5. (b) The m-directing and deactivating groups having a positively charged atom for example : NR + 3 , NH + 3 etc. also attract electrons from the ortho and para positions through strong inductive effect. (c) Th e m-directing and deactivating nature of CCL3 group is due to electron attraction form the o- and p- positions through –I as well as hyperconjugative effect.

6. 3. Ortho and para directing and deactivating groups : Halogens are Ortho and para directing but they deactivate the benzene ring for further electrophilic substitution. This is because there are two opposing effects. They are deactivators because they inductively withdraw electrons from the ring more strongly than they electrons by resonance. Explanation based on carbocation stability: The effect of substituents on orientation and reactivity can be best explained by writing all the possible resonating structure of the carbocation ( arenium ion) intermediate for each of the three possible reaction courses.

7. 1. Ortho, Para directing and activating group : (a) lone pair containing atom attached to ring

8. (b) When alkyl groups are attached to the ring :

9. 2. Meta directing and deactivating group :

10. 3. Ortho, Para directing and deactivating group : (a) Ortho : (b) Para : (c) Meta :

11. Thank you